The present invention relates to methods and arrangements in a telecommunication system, and more particularly to methods and arrangements for supporting measurements on cell-specific reference symbols in mobile telecommunications environments that may or may not include one or more MBMS Single Frequency Networks.
In the forthcoming evolution of the mobile cellular standards like the Global System for Mobile Communication (GSM) and Wideband Code Division Multiple Access (WCDMA), new transmission techniques like Orthogonal Frequency Division Multiplexing (OFDM) are likely to occur. Furthermore, in order to have a smooth migration from the existing cellular systems to the new high-capacity high-data rate system in existing radio spectrum, a new system has to be able to utilize a bandwidth of varying size. A proposal for such a new flexible cellular system, called Third Generation Long Term Evolution (3G LTE), can be seen as an evolution of the 3G WCDMA standard. This system will use OFDM as the multiple access technique (called OFDMA) in the downlink and will be able to operate on bandwidths ranging from 1.25 MHz to 20 MHz. Furthermore, data rates up to 100 Mb/s will be supported for the largest bandwidth. However, it is expected that 3G LTE will be used not only for high rate services, but also for low rate services like voice. Since 3G LTE is designed for Transmission Control Protocol/Internet Protocol (TCP/IP), Voice over IP (VoIP) will likely be the service that carries speech.
The physical layer of a 3G LTE system includes a generic radio frame having a duration of 10 ms. FIG. 1a illustrates one such frame 100 for an LTE Frequency Division Duplex (FDD) system. Each frame has 20 slots (numbered 0 through 19), each slot having a duration of 0.5 ms which normally consists of seven OFDM symbols. A sub-frame is made up of two adjacent slots, and therefore has a duration of 1 ms, normally consisting of 14 OFDM symbols. As LTE downlink transmission is based on OFDM, this means that, within one OFDM symbol, data is transmitted in parallel on a large number of narrowband subcarriers. Thus, the downlink transmission can be described as a time/frequency grid as illustrated in FIG. 1b, in which each resource element or symbol corresponds to one subcarrier during one OFDM symbol. For an LTE system, the spacing between neighboring subcarriers is 15 kHz, and the total number of subcarriers can be as large as 1200 (for the case of a 20 MHz transmission bandwidth). As also illustrated in FIG. 1b, the subcarriers are grouped into resource blocks, wherein each resource block consists of 12 subcarriers during one 0.5 ms slot. With seven OFDM symbols per slot, there is thus a total of 12×7=84 resource elements in a resource block. One such resource block is illustrated as the shaded area in FIG. 1b. 
The radio frame for an LTE Time Division Duplex (TDD) system is similar to that described above for the FDD system, with minor differences. In a TDD system, sub-frames 1 and 7 do not consist of two slots, but rather of three fields (DwPTS, Guard period, and UpPTS). The following discussion, as well as the invention, are applicable to both FDD and TDD systems.
Within each resource block there is a set of resource elements, also known as reference symbols, set to known values. These are illustrated in FIG. 2. Reference symbols can be used by, for example, the User Equipment (UE) to estimate the downlink channel for coherent detection. The reference symbols are also used as part of the LTE mobility function as described below.
As can be seen in FIG. 2, within each resource block there are four reference symbols, two reference symbols within the first OFDM symbol (denoted R1) and two reference symbols in the third from last OFDM symbols (denoted R2). Within the pair of resource blocks corresponding to one sub-frame there are thus a total of eight reference symbols, four reference symbols in the first resource block corresponding to the first slot of the sub-frame and four reference symbols in the second resource block corresponding to the second slot of the sub-frame.
One important aspect of LTE is the mobility function. Hence, procedures are provided for the UE to search for, detect, and synchronize with other cells. To facilitate cell search and synchronization procedures, LTE defines primary and secondary synchronization signals (P-SyS and S-SyS, respectively), which are transmitted on a Primary Synchronization Channel (P-SCH) and a Secondary Synchronization Channel (S-SCH), respectively. The P-SySs and S-SySs are each transmitted twice per frame: once in sub-frame 0, and again in sub-frame 5, as shown in FIG. 1.
The cell-search scheme for LTE can be assumed to consist of the following steps:
1. Detect one out of three possible P-SyS symbols, thereby indicating the 5 ms timing and the cell ID within a currently unknown cell group.
2. Detect frame timing and cell group using the S-SyS. This in combination with the results from step 1 gives an indication of the full cell ID.
3. Use the reference symbols to verify the cell ID. The interested reader is referred to the document R1-062990, entitled “Outcome of cell search drafting session”, TSG-RAN WG1 #46bis, Oct. 9-13, 2006 for more information about this proposal.
4. Read the Broadcast Channel (BCH) to receive cell-specific system information.
Once a cell has been found, the UE can use the measured received power of the reference symbols as input to cell selection and handover decisions.
The LTE system also has modes of operation that utilize an extended cyclic prefix length. When this is the case, each slot includes six rather than seven symbols (i.e., 12 OFDM symbols per sub-frame). While this is less efficient from an overhead standpoint, the longer cyclic prefix may be beneficial in specific environments with very extensive delay spread (e.g., very large cells). The reference symbols are still distributed in the first and third from last OFDM symbols in each slot, but in the case of extended cyclic prefixes in unicast operation, this turns out to be the first and fourth symbols, rather than the first and fifth symbols which is the case for normal cyclic prefix lengths.
In addition to unicast operation, LTE radio access networks also include the possibility of downlink Multimedia Broadcast Multicast Service (MBMS) transmissions using MBMS Single Frequency Network (MBSFN) operation. In LTE, an MBMS Single Frequency Network is implemented by having a number of base stations, or evolved Node Bs (eNodeB), to synchronously transmit identical MBMS information within the same resource block (i.e., same group of subcarriers at the same time) and using identical transport formats (i.e., identical coding rate and modulation scheme). For the case of MBSFN transmission, the transmissions from the different eNodeBs involved in the MBSFN transmission will thus be identical. As a consequence, it will be possible for user equipment to simultaneously receive and utilize the energy of all MBSFN transmissions that are received within the time spanned by an OFDM cyclic prefix. This will significantly improve the MBMS reception quality and thus improve the overall MBMS system performance. The set of cells involved in an MBMS transmission based on MBSFN is referred to as an MBSFN Area.
It should be noted that a single cell may be involved in different MBSFN transmissions corresponding to different sets of cells; that is, different only partly non-overlapping MBSFN areas. Such different MBSFN transmissions corresponding to different MBSFN areas are then taking place in different sub-frames.
In connection with MBSFN operation for transmission of MBMS data, the 3rd Generation Partnership Project (3GPP) has agreed on the definition of a number of concepts. These concepts are illustrated in FIG. 3 and defined as follows:                A Multi-cell MBMS Synchronization Area 301 consists of a group of cells on the same frequency band allocated with contiguous coverage within which area all cells are capable of being synchronized and having the possibility of transmitting MBMS data in MBSFN mode. Multi-cell MBMS Synchronization Areas 301 may be configured independently from MBMS Service Area configurations and are capable of supporting one or more MBSFN Areas (see below for definition). It is permissible to define only one Multi-cell MBMS Synchronization Area 301 for a given geographical area and a given frequency band (i.e. multiple Multi-cell MBMS Synchronization Areas in the same geographical area have to be defined on different frequency bands.)        An MBMS Single Frequency Network Area (MBSFN Area) 303 consists of a group of cells with contiguous coverage areas wherein all of these cells are using the same radio resources (and hence the same frequency band) to synchronously transmit a single MBMS service. The MBSFN area 303 belongs to only one Multi-cell MBMS synchronization area 301. MBSFN Area 303 is composed only of actively transmitting cells at a certain point in time.        The Maximum MBSFN Area 305 is the maximum supported geographical extension of an MBSFN Area 303. It may be limited by the multi-cell MBMS synchronization area 301, the MBMS service area (i.e., the area over which MBMS service is to be provided, possibly by building it up from a number of MBSFN areas 303), and operator configuration.        The MBSFN Guard Area 307 is a group of cells that, due to interference considerations, are restricted from using the same radio resources as those of a nearby MBSFN Area 303.        
LTE allows for both MBSFN transmission and non-MBSFN transmission using the same carrier in what is called “mixed operation.” In mixed operation, some sub-frames are used for MBSFN transmission (so-called “MBSFN sub-frames”), and the remaining sub-frames are used for non-MBSFN transmission (so-called “non-MBSFN sub-frames” or “unicast sub-frames”). However, sub-frames 0 and 5, which include the P-SyS and S-SyS, are always non-MBSFN sub-frames.
Reference symbols are used in the downlink of LTE-systems for demodulation of unicast data and control signalling as well as for measurement purposes. These reference symbols are typically different for neighbour cells (i.e., they are cell specific). However, when an LTE radio access network includes MBSFN transmissions, additional reference symbols are transmitted in sub-frames with MBSFN transmission (i.e., in MBSFN sub-frames). These reference symbols, which can be referred to as MBSFN reference symbols, are identical for all cells involved in the MBSFN transmission (i.e., cell-common). By using the MBSFN reference symbols, the UE can estimate the aggregated channel from all cells involved in the MBSFN transmission. This channel estimate can be used for coherent detection of the combined MBSFN transmission.
FIG. 4 illustrates the overall structure of MBSFN sub-frames in LTE, including the overall reference symbol structure. In this illustration, MBSFN reference symbols are denoted “RM”, and unicast reference symbols are denoted “RU”. In order to minimize the reference symbol overhead, in MBSFN sub-frames unicast references symbols are only transmitted in the first OFDM symbol of the first slot of the sub-frame (an “MBSFN group of OFDM symbols”). Recalling that non-MBSFN sub-frames comprise first and second reference symbols per slot in each of the first and second slots of the sub-frame, it can be seen that the number of unicast reference symbols transmitted in MBSFN sub-frames has been reduced.
In order to determine the channel quality of a cell, (e.g., in conjunction with a handover), user equipment performs measurements on the unicast reference symbols on an “own cell” (i.e., the cell that is presently serving the user equipment) as well as on neighboring cells. To do this in an efficient way, the user equipment needs to know what reference symbols are available in a given sub-frame. As discussed above, for non-MBSFN sub-frames, these reference symbols are transmitted four times per unicast sub-frame, namely in the first and third from last OFDM symbols of each slot.
However, as also discussed above, in sub-frames with MBSFN operation, only the first reference symbol of the first slot will be present. Thus, the set of unicast reference symbols in MBSFN sub-frames can be viewed as a subset of the unicast reference symbols that are present in non-MBSFN sub-frames. Alternatively, the set of unicast reference symbols in non-MBSFN sub-frames can be viewed as an extended set, compared to the set of unicast reference symbols in MBSFN sub-frames.
Thus, unless a user equipment knows that a sub-frame of a given cell is, definitely, a non-MBSFN sub-frame, the user equipment can only use the first reference symbols of the first slot for measurements, as these are the only reference symbols present in all sub-frame types (i.e., both MBSFN and non-MBSFN sub-frames). At the same time, measuring on only the first reference symbol of the first slot leads to reduced measurement performance and is thus undesirable.
Thus, the user equipment should preferably know which sub-frames are non-MBSFN sub-frames in order to be able to utilize the full set of reference symbols of these sub-frames for measurements. For the own cell, this information is available to the user equipment. However, the inventors have recognized that it would also be desirable for the user equipment to know which sub-frames of neighbor cells are non-MBSFN sub-frames in order to be able to utilize the full set of reference symbols of these sub-frames for measurements on these neighbor cells.